If it's the bottom of the basket that keeps detaching, it sounds to me like there's not enough open space for the water to be drawn through the basket, so the pump is actually sucking the bottom off the basket. If the rock takes up a lot of surface area, then I'd try for a smaller rock. Also, make sure you're cleaning the skimmer sock regularly--they catch an awful lot of stuff, and it won't take long for it to clog up so that water can't get through it.